JP4469248B2 - Method for producing high carbon steel rails with excellent wear resistance and ductility - Google Patents

Abstract

Disclosed are methods of producing steel rails having a high carbon content and being excellent in wear resistance and ductility from the slabs for rails. One method involves producing a steel rail having a high content of carbon, comprising finish rolling the rail in two consecutive passes, with a reduction rate per pass of a cross-section of the rail of 2-30%, wherein the conditions of the finish rolling satisfy the following relationship: S ‰¦ 800 / (C × T), wherein S is the maximum rolling interval time (seconds), C is the carbon content of the steel, wherein the carbon content is 0.85-1.40 mass%, and T is the maximum surface temperature (°C) of the rail head. Another method involves producing a steel rail with a high content of carbon, comprising: finish rolling the rail in three or more passes, with a reduction rate per pass of a cross-section of the rail of 2-30%, wherein the conditions of the finish rolling satisfy the following relationship: S ‰¦ 2400 / (C × T × P), wherein S is the maximum rolling interval time (seconds), C is the carbon content of the steel rail, wherein the carbon content is 0.85ˆ¼1.40 mass%, T is the maximum surface temperature (°C) of a rail head, and P is the number of passes, which is 3 or more. In addition to above, controlled additional amounts of V, Nb, N may be added to the steel rail and/or controlled rapid cooling of the rail after rolling may be accomplished to provide further improvements.

Description

  The present invention relates to a method for manufacturing a high carbon steel rail having a pearlite structure for the purpose of simultaneously imparting wear resistance and ductility in a rail used in heavy-duty railways.

High carbon content pearlite steel has been used as a rail material for railways because of its excellent wear resistant steel. However, since the carbon content is very high, there is a problem that ductility and toughness are low.
For example, in an ordinary carbon steel rail having a carbon content of 0.6 to 0.7 mass% shown in Non-Patent Document 1, an impact value at room temperature in a JIS No. 3 U-notch Charpy impact test is about 12 to 18 J / cm ² . When such a rail is used in a low temperature region such as a cold region, there is a problem that a brittle fracture is caused by a minute initial defect or a fatigue crack.
Further, in recent years, rail steel has been further increased in carbon to improve wear resistance, resulting in a problem that ductility and toughness are further reduced.

  In general, to improve the ductility and toughness of pearlite steel, it is effective to refine the pearlite structure (pearlite block size), specifically, to refine the austenite structure before pearlite transformation and to refine the pearlite structure. It is said. In order to achieve the fine graining of the austenite structure, a reduction in rolling temperature during hot rolling, an increase in rolling reduction, and a heat treatment by low-temperature reheating after rail rolling are performed. In order to refine the pearlite structure, pearlite transformation is promoted from the austenite grains using transformation nuclei.

  However, in the production of rails, from the viewpoint of securing formability during hot rolling, there are limits to the reduction in rolling temperature and the increase in reduction, and sufficient austenite grain refinement could not be achieved. In addition, for pearlite transformation from austenite grains using transformation nuclei, there are problems such as difficulty in controlling the amount of transformation nuclei and instability of pearlite transformation from within grains. Could not be achieved.

  In order to drastically improve the ductility and toughness of a pearlite structure rail due to these problems, a method of refining the pearlite structure by performing low-temperature reheating after rail rolling and then performing pearlite transformation by accelerated cooling is a method. Has been used. However, in recent years, the carbon of rails has been increased to improve wear resistance, and during the low temperature reheating treatment, coarse carbides remain undissolved in the austenite grains, reducing the ductility and toughness of the pearlite structure after accelerated cooling. There was a problem. Moreover, since it is reheating, there also existed economical problems, such as high manufacturing cost and low productivity.

Accordingly, development of a method for producing a high carbon steel rail that ensures formability during rolling and that refines the pearlite structure after rolling has been demanded. In order to solve this problem, a method for producing a high carbon steel rail as described below has been developed.
(1) A method for producing a high ductility rail in which rolling of a high-carbon steel-containing steel rail is performed for three or more consecutive passes in a predetermined time between passes (Patent Document 1).
(2) In the finish rolling of steel rails containing high carbon steel, high wear resistance is achieved in which rolling is performed for two or more consecutive passes at a predetermined time between passes, and further, accelerated cooling is performed after completion of rolling after continuous rolling. Manufacturing method of high-strength and high-toughness rail (Patent Document 2).
(3) A method of manufacturing a highly wear-resistant, high-toughness rail in which high-carbon steel-containing steel rails are subjected to cooling between passes, followed by continuous rolling, followed by accelerated cooling after rolling. Law (Patent Document 3).

The characteristics of these rails are that, in order to improve the ductility and toughness of the rails, as a method of refining the pearlite structure, we examined the refining of the austenite structure, and the high carbon steel has a relatively low temperature and a small reduction amount. Utilizing the fact that it is easy to recrystallize, fine-sized austenite grains are obtained by continuous rolling under a small pressure, and the ductility and toughness of pearlite steel are improved.
Japanese Unexamined Patent Publication No. 7-173530 JP 2001-234238 A JP 2002-226915 A JISE1101-1990

In the continuous rolling method shown above, depending on the combination of the carbon content of steel, the temperature during continuous hot rolling, the number of rolling passes and the time between passes, the austenite structure cannot be refined and the pearlite structure is coarse. There was a problem that the ductility was not improved.
Especially in steel with high carbon content, the grain growth rate immediately after rolling is large, so depending on the selection of the time between passes during continuous rolling, the growth of austenite grains between passes becomes significant, and the continuous rolling shown above Even if cooling is performed between methods and passes, there is a problem that the austenite structure cannot be refined, the pearlite structure becomes coarse, and the ductility does not improve.
Against such a background, in the finish rolling of steel rails containing high carbon steel, rail production that obtains finely grained fine austenite grains and at the same time suppresses the growth of austenite grains between passes and stably improves ductility. There was a need to develop a method.

  That is, in the present invention, when continuous hot rolling is performed using a steel piece having a high carbon content as a rail, the maximum time between passes (S, sec) is set to the carbon content (C, mass%) of the steel, and the maximum rail head during rolling. Surface temperature (T, ° C.), and further below the value calculated from the number of passes (P, times), then accelerated to the rail head surface in a certain temperature range after continuous hot rolling The purpose of this is to obtain cooling and to obtain a fine pearlite structure with high hardness, and to ensure the wear resistance and ductility of the rail head.

The present invention has the following configuration.
(1) In mass%,
C: more than 0.85 to 1.40% ,
Si: 0.05 to 2.00%,
Mn: 0.05 to 2.00%
In addition,
Cr: 0.05 to 2.00%,
Mo: 0.01 to 0.50%,
B: 0.0001 to 0.0050%,
Co: 0.003 to 2.00%,
Cu: 0.01 to 1.00%,
Ni: 0.01-1.00%,
Ti: 0.0050-0.0500%,
Mg: 0.0005 to 0.0200%,
Ca: 0.0005 to 0.0150%,
Al: 0.0100-1.00%,
Zr: 0.0001 to 0.2000%,
N: 0.0060-0.0200%,
V: 0.005-0.50%,
Nb: 0.002 to 0.050%
When manufacturing a rail from rail rolling steel slabs containing one or more of the following, and the balance Fe and unavoidable impurities, the number of consecutive passes is 2 to 30% with a cross-section reduction rate per pass of 2 to 30%. In finish rolling, the time between rolling passes (S, sec) is a value represented by the following formula 1 consisting of the carbon content (C, mass%) of steel and the maximum rail head surface temperature (T, ° C) during rolling. against (CPTl), method for producing high-carbon steel rail you and performing continuous rolling such that S ≦ CPTl.
CPT1 = 800 / (C × T) (1)

(2) In mass%,
C: more than 0.85 to 1.40%,
Si: 0.05 to 2.00%,
Mn: 0.05 to 2.00%
In addition,
Cr: 0.05 to 2.00%,
Mo: 0.01 to 0.50%,
B: 0.0001 to 0.0050%,
Co: 0.003 to 2.00%,
Cu: 0.01 to 1.00%,
Ni: 0.01-1.00%,
Ti: 0.0050-0.0500%,
Mg: 0.0005 to 0.0200%,
Ca: 0.0005 to 0.0150%,
Al: 0.0100-1.00%,
Zr: 0.0001 to 0.2000%,
N: 0.0060-0.0200%,
V: 0.005-0.50%,
Nb: 0.002 to 0.050%
When manufacturing a rail from a rail rolling steel slab comprising one or more of the following, and the balance Fe and inevitable impurities, the cross-section reduction rate per pass is 2 to 30%, and the number of passes is 3 times or more. In continuous finish rolling, the maximum time between rolling passes (S, sec) is the carbon content of steel (C, mass%), the maximum rail head surface temperature (T, ° C) during rolling, and the number of passes (P, times). A method for producing a high carbon steel rail, characterized in that continuous rolling is performed so that S ≦ CPT2 is satisfied with respect to a value (CPT2) represented by the following formula 2 comprising:
CPT2 = 2400 / (C × T × P) (Equation 2)

(3) In the chemical composition of the steel rail, the value (PC) represented by the following formula 3 is in the range of 0.30 ≧ PC ≧ 0.04. (1) or (2) The manufacturing method of the high carbon steel rail of description .
PC = V (mass%) + 10 × Nb (mass%) + 5 × N (mass%) (Equation 3)
(4) Immediately after hot continuous rolling, the rail head surface is accelerated and cooled to 950 to 750 ° C. at a cooling rate of 5 to 30 ° C./sec, any one of (1) to (3) above The manufacturing method of the high carbon steel rail of description .
(5) The electrolyte after hot continuous rolling, the subsequently 700 ° C. or higher, the rail head surface accelerated cooling to at least 600 ° C. at a cooling rate 2 to 30 ° C. / sec, characterized by subsequently cooled ( The manufacturing method of the high carbon steel rail of any one of 1)-(4) .

  According to the present invention, in the production of steel rails containing high carbon, the maximum rolling pass time during continuous hot rolling is calculated from the amount of steel carbon, the maximum rail head surface temperature during rolling, and the number of passes. In addition to this, the addition amount of V, Nb, and N is controlled, the austenite grains of the rail head are refined, and after rolling, the rail head surface is cooled to a predetermined temperature range within a predetermined temperature range. By performing accelerated cooling at a high speed, it is possible to obtain a fine pearlite structure with high hardness, improve the ductility of the rail head, and improve the service life.

The present invention is described in detail below.
First, the inventors of the present invention, in rail steel containing high carbon, by the combination of the amount of carbon of the steel, the rail head surface temperature during continuous finish rolling, the cross-section reduction rate and the time between passes, by the coarsening of the pearlite structure, The factors that did not improve the ductility were analyzed. As a result of various verification experiments, it was confirmed that austenite grains after hot continuous rolling coarsen when the maximum time between passes during a continuous rolling exceeds a certain value.

  Therefore, the present inventors have analyzed the cause of austenite grain coarsening due to an increase in the maximum interpass time in a high carbon content rail steel. As a result of various verification experiments, it was confirmed that the grain growth of austenite grains had a positive correlation with the carbon content of steel and the maximum rail head surface temperature during continuous finish rolling. As a result of continuing the analysis, it was confirmed that there is a positive correlation with the number of rolling passes in continuous finish rolling with three or more passes.

  From these examination results, the present inventors have correlated the relationship between the optimal time between passes that does not coarsen austenite grains and the carbon content of the steel, the maximum rail head surface temperature during continuous finish rolling, and the number of rolling passes. The analysis was performed. As a result, if the number of passes of continuous finish rolling is 2, the formula consisting of the amount of steel carbon and the maximum rail head surface temperature during continuous finish rolling, if the number of passes of continuous finish rolling is 3 or more, The austenite grains between rolling passes are controlled by controlling the maximum interpass time during continuous rolling below the value calculated by the formula consisting of the carbon content of steel, the maximum rail head surface temperature during continuous finish rolling, and the number of passes. It was confirmed that grain growth of the austenite was suppressed and the austenite grains after hot continuous rolling were refined.

Next, the present inventors examined a method for suppressing grain growth of austenite grains generated after continuous rolling by applying precipitates. As a result of the hot continuous rolling experiment, V carbide, V nitride, V carbonitride, Nb carbide, and Nb carbonitride precipitate during continuous rolling, and pinning suppresses grain growth of austenite grains after rolling. I found out.
Furthermore, the present inventors examined conditions under which V carbide, V nitride, V carbonitride, Nb carbide, and Nb carbonitride effective for pinning austenite grains are sufficiently precipitated. As a result, the austenite grains after continuous rolling are controlled by controlling the addition amounts of V, Nb, and N within the range calculated by the formula consisting of the addition amounts (mass%) of V, Nb, and N. It was confirmed that the growth was sufficiently suppressed.

Next, the present inventors examined a method for suppressing grain growth of austenite grains generated after continuous rolling by applying accelerated cooling immediately after rolling. As a result, it has been found that immediately after the above-described continuous rolling, the rail head surface is subjected to accelerated cooling at a cooling temperature within a predetermined temperature range and a predetermined range, thereby suppressing grain growth of the austenite grains after rolling.
In addition to these inventions, the present inventors examined a method for obtaining a pearlite structure excellent in wear resistance and ductility from fine austenite grains. As a result, by accelerating and cooling the rail head surface in the austenite region at a predetermined temperature range and a predetermined cooling rate, a high hardness and fine pearlite structure can be obtained, and the wear resistance and ductility of the rail head can be improved. It was found that it can be secured.

  Therefore, in the present invention, when continuously rolling steel slab containing high carbon as a rail, the maximum time between passes is a formula comprising the carbon amount of steel and the maximum rail head surface temperature during rolling, or the carbon amount of steel, In order to control the maximum rail head surface temperature during rolling and the value calculated by the formula consisting of the number of passes, in addition to this, in order to suppress the grain growth of austenite grains generated after continuous rolling, V, Nb, N In addition, the rail head surface is accelerated and cooled at a predetermined temperature range and at a predetermined cooling rate immediately after continuous rolling, and further, wear resistance is controlled. In order to obtain a pearlite structure excellent in ductility, the surface of the rail head in the austenite region is accelerated and cooled at a predetermined temperature range and a predetermined cooling rate, thereby obtaining a high hardness and fine pearlite structure. Wear resistance Improvement of the security and ductility has been found that can be achieved.

  That is, in the present invention, when continuous hot rolling is performed using a high carbon content steel slab as a rail, considering the carbon content of the steel, the maximum rail head surface temperature during rolling, and the number of passes, the maximum gap between rolling By controlling the time, the austenite grains of the rail head are refined, and in addition to this, the addition amount of V, Nb, and N is controlled, and after rolling, the rail head surface is moved to a predetermined temperature range and a predetermined temperature range. The present invention relates to a method for producing a high carbon steel rail for the purpose of simultaneously ensuring wear resistance and ductility of a rail head by performing accelerated cooling at a cooling rate.

Next, the reason for limitation of the present invention will be described in detail.
(1) Reasons for limiting the chemical composition of steel rails:
First, the reason why the chemical components of the rail steel are limited to the above claims will be described in detail.
C is an effective element that promotes pearlite transformation and ensures wear resistance. If the C content is 0.85% or less, the volume ratio of the cementite phase in the pearlite structure cannot be secured, and the wear resistance cannot be maintained in heavy-duty railways. On the other hand, if the C content exceeds 1.40%, a large amount of proeutectoid cementite structure is formed at the prior austenite grain boundaries in this production method, and the wear resistance and ductility deteriorate. For this reason, the amount of C was limited to more than 0.85 to 1.40%. When the carbon content is 0.95% or more, the wear resistance is further improved, and the effect of improving the service life of the rail is high.

  In addition, the rail manufactured with the above component composition improves the hardness (strengthening) of the pearlite structure, improves the ductility of the pearlite structure, prevents softening of the weld heat affected zone, and controls the cross-sectional hardness distribution inside the rail head. For the purpose, Si, Mn, Cr, Mo, V, Nb, B, Co, Cu, Ni, Ti, Mg, Ca, Al, Zr, and N are added as necessary.

  Here, Si is an element that increases the hardness (strength) of the rail head by solid solution strengthening in the ferrite phase, suppresses the formation of proeutectoid cementite structure, and secures hardness and ductility. Mn is an element that secures the hardness of the pearlite structure by increasing the hardenability and reducing the pearlite lamella spacing. Cr and Mo secure the hardness of the pearlite structure by raising the equilibrium transformation point of pearlite and mainly reducing the pearlite lamella spacing. V and Nb suppress the growth of austenite grains by carbides and nitrides generated by hot rolling and the subsequent cooling process, and further improve the ductility and hardness of the pearlite structure by precipitation hardening. In addition, carbides and nitrides are stably generated during reheating, and softening of the weld joint heat-affected zone is prevented.

  B refines the formation of proeutectoid cementite structure, reduces the dependency of the pearlite transformation temperature on the cooling rate, improves the ductility of the rail, and makes the hardness distribution of the rail head uniform. Co is an element that dissolves in the ferrite in the pearlite structure to increase the hardness of the pearlite structure, and at the same time promotes the formation of a fine ferrite structure on the wear surface of the rail head and improves the wear resistance. Cu dissolves in the ferrite in the pearlite structure and increases the hardness of the pearlite structure. Ni prevents embrittlement during hot rolling due to addition of Cu, simultaneously improves the hardness of the pearlite steel, and further prevents softening of the heat affected zone of the weld joint.

Ti refines the structure of the heat-affected zone and prevents embrittlement of the weld joint. Mg and Ca make austenite grains finer during rail rolling, and at the same time promote pearlite transformation and improve the ductility of the pearlite structure. Al moves the eutectoid transformation temperature to the high temperature side, strengthens the pearlite structure, and improves the wear resistance of the rail. Furthermore, the amount of eutectoid carbon is moved to the higher carbon side to suppress the formation of proeutectoid cementite structure. Zr suppresses the formation of a segregation zone at the center of the slab by increasing the equiaxed crystallization rate of the solidified structure by the inclusion of ZrO ₂ inclusions as the solidification nucleus of the high carbon rail steel. To reduce the ductility of the rail. N is mainly intended to improve ductility by promoting pearlite transformation from the austenite grain boundary and making the pearlite structure fine.

The reasons for limiting these components will be described in detail below.
Si is an essential component as a deoxidizing material. Moreover, it is an element which raises the hardness (strength) of a rail head by the solid solution strengthening to the ferrite phase in a pearlite structure | tissue. Furthermore, in hypereutectoid steel, it is an element that suppresses the formation of proeutectoid cementite structure and suppresses the decrease in ductility. However, when the Si content is less than 0.05%, these effects cannot be sufficiently expected. On the other hand, if the amount of Si exceeds 2.00%, a lot of surface defects are generated during hot rolling, and weldability deteriorates due to generation of oxides. Further, the hardenability is remarkably increased, and a martensite structure that is harmful to the wear resistance and ductility of the rail is generated. For this reason, the amount of Si was limited to 0.05 to 2.00%.

  Mn is an element that increases the hardenability and refines the pearlite lamella spacing to ensure the hardness of the pearlite structure and improve the wear resistance. However, if the amount of Mn is less than 0.05%, the effect is small, and it is difficult to ensure the wear resistance required for the rail. On the other hand, if the amount of Mn exceeds 2.00%, the hardenability is remarkably increased, and a martensite structure that is harmful to wear resistance and ductility is easily generated. For this reason, the amount of Mn was limited to 0.05 to 2.00%.

  Cr raises the equilibrium transformation point of pearlite and, as a result, refines the pearlite structure and contributes to higher hardness (strength), and at the same time, strengthens the cementite phase and improves the hardness (strength) of the pearlite structure. It is an element that improves wear resistance. However, the effect is small when the Cr content is less than 0.05%. On the other hand, when the Cr content exceeds 2.00%, the hardenability is remarkably increased, a large amount of martensite structure is generated, and the wear resistance and ductility of the rail are lowered. For this reason, the Cr content is limited to 0.05 to 2.00%.

  Mo, like Cr, is an element that increases the equilibrium transformation point of pearlite and, as a result, refines the pearlite structure, thereby contributing to higher hardness (strength) and improving the hardness (strength) of the pearlite structure. If the amount of Mo is less than 0.01%, the effect is small, and the effect of improving the hardness of the rail steel is not seen at all. On the other hand, if the Mo content exceeds 0.50%, the transformation rate of the pearlite structure is remarkably reduced, and a martensite structure that is harmful to ductility is easily generated. For this reason, Mo addition amount was limited to 0.01 to 0.50%.

  B forms iron boride at the prior austenite grain boundary, refines the formation of proeutectoid cementite structure, reduces the cooling rate dependence of the pearlite transformation temperature, and makes the head hardness distribution uniform. It is an element that prevents a decrease in the ductility of the rail and extends its life. However, if the amount of B is less than 0.0001%, the effect is not sufficient, and no improvement is observed in the formation of proeutectoid cementite structure and the hardness distribution of the rail head. On the other hand, if the amount of B exceeds 0.0050%, coarse iron carbon borides are formed at the prior austenite grain boundaries, and the ductility of the rail and further the fatigue damage resistance are greatly reduced. It was limited to 0001 to 0.0050%.

  Co is an element that dissolves in ferrite in the pearlite structure and improves the hardness (strength) of the pearlite structure by solid solution strengthening, and further increases the transformation energy of the pearlite to make the pearlite structure finer. It is an element that improves ductility. Furthermore, it is an element that further refines the fine ferrite structure formed by contact with the wheel on the wear surface of the rail head and improves the wear resistance. However, if the Co content is less than 0.003%, the effect cannot be expected. On the other hand, if the Co content exceeds 2.00%, the ductility of the ferrite phase in the pearlite structure is remarkably lowered, spalling damage occurs on the rolling surface, and the surface damage resistance of the rail is lowered. For this reason, the amount of Co was limited to 0.003 to 2.00%.

  Cu is an element that dissolves in the ferrite in the pearlite structure and improves the hardness (strength) of the pearlite structure by solid solution strengthening. However, if the amount of Cu is less than 0.01%, the effect cannot be expected. On the other hand, if the Cu content exceeds 1.00%, a martensite structure that is harmful to wear resistance is likely to be generated due to a significant improvement in hardenability. Furthermore, the ductility of the ferrite phase in the pearlite structure is remarkably lowered, and the ductility of the rail is lowered. For this reason, the amount of Cu was limited to 0.01 to 1.00%.

Ni is an element that prevents embrittlement during hot rolling due to the addition of Cu, and at the same time, increases the hardness (strength) of pearlite steel by solid solution strengthening to ferrite. Further, in the weld heat affected zone, an intermetallic compound of Ni ₃ Ti that is composited with Ti is finely precipitated and is an element that suppresses softening by precipitation strengthening. However, if the amount of Ni is less than 0.01%, the effect is remarkably small. On the other hand, when the Ni content exceeds 1.00%, the ductility of the ferrite phase is remarkably lowered, spalling damage is generated on the rolling surface, and the surface damage resistance of the rail is lowered. For this reason, the amount of Ni was limited to 0.01 to 1.00%.

  By utilizing the fact that Ti carbide and Ti nitride precipitated during reheating during welding do not dissolve, the structure of the heat-affected zone heated to the austenite region is refined and brittleness of the welded joint is achieved. It is an effective ingredient for preventing oxidization. However, the effect is small when the Ti content is less than 0.0050%. On the other hand, if the Ti content exceeds 0.0500%, coarse Ti carbides and Ti nitrides are generated, and the ductility of the rail and, in addition to this, the fatigue damage resistance is greatly reduced. Limited to .0050-0.0500%.

  Mg combines with O, S, Al, or the like to form fine oxides, suppresses grain growth during reheating during rail rolling, refines austenite grains, It is an effective element for improving ductility. Furthermore, MgO and MgS finely disperse MnS, forming a Mn dilute band around MnS, contributing to the generation of pearlite transformation, and as a result, reducing the pearlite block size, thereby reducing the ductility of the pearlite structure. It is an effective element to improve. However, if the amount of Mg is less than 0.0005%, the effect is weak. On the other hand, if the Mg amount exceeds 0.0200%, a coarse Mg oxide is generated, and the ductility of the rail and further fatigue damage resistance is lowered. Therefore, the Mg amount is limited to 0.0005 to 0.0200%. .

  Ca has a strong binding force with S, forms a sulfide as CaS, CaS finely disperses MnS, forms a Mn dilute band around MnS, and contributes to the generation of pearlite transformation. As a result, it is an effective element for improving the ductility of the pearlite structure by reducing the pearlite block size. However, if the Ca content is less than 0.0005%, the effect is weak. Further, when the Ca content exceeds 0.0150%, a coarse oxide of Ca is generated, and the ductility of the rail and further the fatigue damage resistance are lowered. Therefore, the Ca content is limited to 0.0005 to 0.0150%. .

  Al is an essential component as a deoxidizing material. In addition, it is an element that moves the eutectoid transformation temperature to the higher temperature side and the amount of eutectoid carbon to the higher carbon side, and is an effective element for increasing the strength of the pearlite structure and suppressing the formation of the proeutectoid cementite structure. However, when the Al content is less than 0.0100%, the effect is weak. Also, if the Al content exceeds 1.00%, it becomes difficult to make a solid solution in the steel, and coarse alumina inclusions that become the starting point of fatigue damage are generated, and the ductility of the rail and further the fatigue damage resistance are improved. descend. Further, since oxides are generated during welding and weldability is remarkably lowered, the Al content is limited to 0.0100 to 1.00%.

Zr has good lattice matching with γ-Fe because ZrO ₂ inclusions have good lattice matching with γ-Fe, so that γ-Fe becomes a solidification nucleus of high-carbon rail steel that is a solidification primary crystal, and increases the equiaxed crystallization rate of the solidification structure An element that suppresses the formation of a segregation zone at the center of a slab and suppresses the formation of a pro-eutectoid cementite structure generated in a rail segregation portion. However, if the amount of Zr is less than 0.0001%, the number of ZrO ₂ inclusions is small and does not exhibit a sufficient effect as a solidification nucleus. As a result, a pro-eutectoid cementite structure is generated in the segregation part, and the ductility of the rail is lowered. If the amount of Zr exceeds 0.2000%, a large amount of coarse Zr-based inclusions are produced, and the ductility of the rail is reduced, and fatigue damage starting from coarse Zr-based inclusions is likely to occur. The service life of the battery is reduced. For this reason, the amount of Zr was limited to 0.0001 to 0.2000%.

N precipitates V nitride, V carbonitride, and Nb carbonitride during continuous rolling, and suppresses austenite grain growth. In addition, V nitride, V carbonitride, and Nb carbonitride are precipitated in the cooling process after continuous rolling to increase the ductility of the pearlite structure and at the same time improve the hardness (strength). Furthermore, in the heat-affected zone reheated to a temperature range below the Ac1 point, an element effective for precipitating V nitride, V carbonitride and Nb carbonitride and preventing softening of the weld joint heat-affected zone. It is. In addition, it is an element effective for improving the ductility of the pearlite structure by promoting the pearlite transformation from the austenite grain boundary by segregating to the austenite grain boundary and by reducing the pearlite block size.
However, if the N content is less than 0.0060%, the effect is weak. Further, if the N amount exceeds 0.0200%, it becomes difficult to make a solid solution in the steel, and bubbles are generated as the starting point of fatigue damage. Therefore, the N amount is limited to 0.0060 to 0.0200%. .
In the rail steel, N is contained as a maximum of about 0.0050% as an impurity. Therefore, to obtain the above effect, N needs to be added in an amount of at least 0.0060%.

  V suppresses the growth of austenite grains by V carbide, V nitride, and V carbonitride precipitated during continuous rolling, and V carbide, V nitride, and V carbon precipitated during the cooling process after continuous rolling. It is an element effective for improving the hardness (strength) as well as enhancing the ductility of the pearlite structure by precipitation hardening with nitride. Furthermore, in the heat-affected zone reheated to a temperature range below the Ac1 point, V carbide, V nitride, and V carbonitride precipitate in a relatively high temperature range, thereby preventing the weld joint heat affected zone from being softened. Is an effective element. However, if the amount of V is less than 0.005%, the effect cannot be sufficiently expected, and no improvement in the ductility and hardness of the pearlite structure is observed. On the other hand, if the V content exceeds 0.500%, coarse V carbide, V nitride, and V carbonitride that are the starting points of fatigue damage are generated, and the ductility and fatigue damage resistance of the rail are lowered. Therefore, the V amount is limited to 0.005 to 0.500%.

  Nb suppresses grain growth of austenite grains by Nb carbide and Nb carbonitride precipitated during continuous rolling, and pearlite by precipitation hardening by Nb carbide and Nb carbonitride precipitated in the cooling process after continuous rolling. It is an element effective for enhancing the hardness (strength) as well as increasing the ductility of the structure. In addition, Nb carbide and Nb carbonitride precipitate from the low temperature range to the high temperature range in the heat affected zone reheated to a temperature range below the Ac1 point, and are effective in preventing softening of the weld joint heat affected zone. Element. However, the effect cannot be expected when the Nb content is less than 0.002%, and no improvement in the hardness or ductility of the pearlite structure is observed. On the other hand, if the Nb content exceeds 0.050%, coarse Nb carbide or Nb carbonitride that becomes the starting point of fatigue damage is generated, and the ductility and fatigue damage resistance of the rail are lowered. For this reason, the amount of Nb was limited to 0.002 to 0.050%.

(2) Reasons for limiting the addition amounts of V, Nb, and N that suppress the grain growth of austenite grains after rolling: Next, the addition amounts of V, Nb, and N are set to V (mass%) and Nb (mass%), respectively. ) And N (mass%) will be explained for the reason limited to the range calculated by the following formula (formula 3).
In continuous rolling of high carbon content rail steel, a method of suppressing grain growth of austenite grains generated after rolling was studied by applying precipitates. As a result, it has been found that V carbide, V nitride, V carbonitride, Nb carbide, and Nb carbonitride precipitate during continuous rolling, and the grain growth of austenite grains after rolling is suppressed by pinning.
Next, the conditions under which V carbide, V nitride, V carbonitride, Nb carbide, and Nb carbonitride effective for pinning of austenite grains were sufficiently examined were examined. As a result, it was confirmed that the formation of these precipitates had a positive correlation with the amounts of V, Nb, and N added.

From these results, the range of the amount of V, Nb, and N added to sufficiently suppress the grain growth of austenite grains was examined by experiments. As a result, the addition amount of V (mass%) and Nb (mass%), and the addition amount of N (mass%) added to promote the formation of nitrides of V and carbonitrides of V and Nb Were confirmed to have different contribution ratios per unit addition amount, and the contribution ratios were obtained by experiments, and the following formula (formula 3) was derived.
Furthermore, from this equation, the optimum range of addition amounts of V, Nb, and N was examined by experiments. As a result, by controlling the PC value obtained from the following formula (formula 3) within the range of 0.30 ≧ PC ≧ 0.04, the grain growth of austenite grains after continuous rolling is sufficiently suppressed. I found out.
When the PC value is out of the above range due to the combination of the addition amounts of V, Nb, and N, grain growth of the austenite grains after continuous rolling is not sufficiently suppressed, and the ductility of the pearlite structure after heat treatment Does not improve sufficiently. Therefore, even if the PC value is out of the above range, the ductility cannot be improved, but it does not adversely affect the characteristics of the rail.
PC = V (mass%) + 10 × Nb (mass%) + 5 × N (mass%) (Equation 3)

Here, N is added to promote the formation of V nitride, V carbonitride, and Nb carbonitride, and the above precipitates are not formed when N alone is added, and austenite grain growth occurs. There is no suppression effect. Therefore, in order to obtain the above effect of suppressing grain growth of austenite grains, V alone, Nb alone, composite addition of V and Nb, composite addition of V and N, composite addition of Nb and N, composite of V, Nb and N Only in the case of addition, in the calculation of the above formula (formula 3), the amount of N added alone is treated as 0 (mass%).
As explained above for the reason for limiting the amount of N added, in the rail steel, N is included as a maximum of about 0.0050% as an impurity, but formation of V nitride, V carbonitride, and Nb carbonitride. In order to promote N, addition of at least 0.0060% or more is necessary. Therefore, in the calculation of the above formula (formula 3), the amount of N at the impurity level is treated as 0 (mass%).

(3) Reason for limiting the cross-section reduction rate per pass:
The reason why the cross-section reduction rate per pass during finish rolling is limited to the range of 2 to 30% will be described. If the cross-section reduction rate per pass during finish rolling exceeds 30%, the heat generation amount after hot rolling is large, the head surface temperature after rolling is greatly increased, and the austenite grains of the head are coarsened. The ductility of the rail cannot be secured. Furthermore, it becomes difficult to ensure formability in rail rolling. Also, if the cross-sectional reduction rate per pass during finish rolling is less than 2%, the minimum amount of strain required to recrystallize the austenite grains in the rail head cannot be secured, and the austenite grains do not become finer. The ductility of the rail cannot be ensured. For this reason, the cross-sectional reduction rate per pass at the time of finish rolling is limited to a range of 2 to 30%.

(4) Reason for limitation of maximum rolling pass time:
The maximum rolling pass time (S) at the time of continuous rolling is the following consisting of the carbon content (C, mass%) of steel, the maximum rail head surface temperature (T, ° C) at the time of rolling, and the number of passes (P, times). The reason for limiting to below the value calculated by the formula (1 formula, 2 formula) will be described.
As a result of analyzing the cause of coarsening of austenite grains due to the increase in the maximum rolling pass time in high-carbon rail steel, the austenite grain growth is the carbon content of the steel, the maximum rail head during continuous finish rolling. It was confirmed that there was a positive correlation with the surface temperature. As a result of continuing the analysis, it was confirmed that there is a positive correlation with the number of rolling passes in continuous finish rolling with three or more passes.

From these results, the optimal maximum rolling pass time and carbon content (C, mass%), maximum rail head surface temperature (T, ° C) during continuous finish rolling, and number of rolling passes that do not coarsen austenite grains. The relationship with (P, times) was analyzed by multiple correlation. As a result, when the number of passes of continuous finish rolling is 2, the following formula (1 formula) consisting of the carbon content of steel and the maximum rail head surface temperature during continuous finish rolling, the number of passes of continuous finish rolling is 3 In the case of more than the number of turns, the carbon content of steel, the maximum rail head surface temperature during continuous finish rolling, and the value (CPT1, CPT2) calculated by the following formula (2 formulas) consisting of the number of passes are below the values during continuous rolling. It has been found that by controlling the maximum time between rolling passes (S, sec), the growth of austenite grains between rolling passes is suppressed and the austenite grains after hot continuous rolling are refined.
CPT1 = 800 / (C × T) (1)
CPT2 = 2400 / (C × T × P) (Equation 2)
S (sec) ≤ CPT1, CPT2

Here, the maximum time between paths is defined. In the present invention, the maximum time between passes indicates the maximum value of the elapsed time during hot rolling for each pass in continuous rolling. Therefore, in the case of three-pass continuous rolling, the larger one of the elapsed time of the first pass and the second pass and the elapsed time of the second pass and the third pass is handled as the maximum time between passes.
In Formulas 1 and 2, C is the amount of carbon (mass%), T is the maximum rail head surface temperature (° C.) during continuous finish rolling, and P is the number of rolling passes (times). Both values are positive values, and P is a natural number.
Moreover, the maximum rail head surface temperature at the time of rolling is the maximum value on the rail head surface in each pass in the case of continuous rolling.

(5) Reason for limiting head accelerated cooling conditions immediately after hot rolling:
The reason why the accelerated cooling rate and accelerated cooling stop temperature of the rail head surface immediately after the hot continuous rolling are limited to the above claims will be described in detail.
First, the range of the accelerated cooling rate will be described. When the cooling rate of the rail head surface after hot continuous rolling is less than 5 ° C./sec, austenite grains become coarse during accelerated cooling, and ductility of the rail head decreases. Also, if the cooling speed of the rail head surface exceeds 30 ° C / sec, the amount of recuperation from the inside of the rail head generated after accelerated cooling is large and the temperature of the rail head surface rises, so the austenite grains become coarse and the rail Head ductility is reduced. For this reason, the range of the accelerated cooling rate on the rail head surface is limited to the range of 5 to 30 ° C./sec.

  Next, the range of the accelerated cooling temperature will be described. When accelerated cooling of the rail head surface is stopped at a temperature exceeding 950 ° C., depending on the amount of carbon in the steel, austenite grains grow markedly after the accelerated cooling is completed, and the austenite grains become coarse and ductility of the rail head decreases. To do. Also, when accelerated cooling of the rail head surface to below 750 ° C., depending on the cooling rate, the amount of recuperated heat from the inside of the rail head generated after accelerated cooling increases, and the temperature of the rail head surface rises, so austenite grains It becomes coarse and the duct head becomes less ductile. For this reason, it was limited to performing accelerated cooling to 950-750 degreeC.

(6) Reason for limitation of head accelerated cooling condition after hot rolling:
The reason why the accelerated cooling rate and the accelerated cooling stop temperature of the rail head surface in the final heat treatment performed after the hot continuous rolling are limited to the above claims will be described in detail.
First, the accelerated cooling rate start temperature will be described. When the accelerated cooling rate start temperature on the rail head surface is less than 700 ° C., pearlite transformation starts before accelerated cooling, the high hardness of the rail head cannot be achieved, and the wear resistance cannot be ensured. Depending on the carbon content and alloy composition of the steel, a pro-eutectoid cementite structure is formed, and the ductility of the rail head surface is reduced. For this reason, the accelerated cooling rate start temperature of the rail head surface was set to 700 ° C. or higher.

  Next, the range of the accelerated cooling rate will be described. If the accelerated cooling rate on the surface of the rail head is less than 2 ° C./sec, the high hardness of the rail head cannot be achieved under these rail manufacturing conditions, and it becomes difficult to ensure the wear resistance of the rail head. Furthermore, depending on the carbon content and alloy composition of the steel, a pro-eutectoid cementite structure is formed, and the ductility of the head of the rail is lowered. On the other hand, when the accelerated cooling rate exceeds 30 ° C./sec, a martensitic structure is generated in this component system, and the ductility of the rail head is greatly reduced. For this reason, the range of the accelerated cooling rate of the rail head surface was limited to the range of 2 to 30 ° C./sec.

Next, the range of the accelerated cooling temperature will be described. If the accelerated cooling of the rail head is stopped at a temperature exceeding 600 ° C., excessive recuperation occurs from the inside of the rail after the accelerated cooling is completed. As a result, the pearlite transformation temperature rises due to the temperature rise, the pearlite structure cannot have a high hardness, and the wear resistance cannot be ensured. In addition, the pearlite structure becomes coarse and the duct head ductility also decreases. For this reason, it was limited to perform accelerated cooling to at least 600 ° C.
The lower limit of the temperature at which the accelerated cooling of the rail head is terminated is not particularly limited, but the hardness of the rail head surface is ensured, and the generation of martensite structure that is easy to be generated in the segregated portion inside the head is generated. In order to prevent this, the lower limit is substantially 400 ° C.

Here, the part of the rail will be described. FIG. 1 shows the names of the rail parts. The “rail head” is a portion including the top (code: 1) and the head corner (code: 2) shown in FIG. Rail head surface temperature during rolling can be achieved by controlling the temperature of the head surface of the top (code: 1) and the head corner (code: 2), thereby reducing the austenite grain size during rolling, The ductility of the rail can be improved.
Further, the accelerated cooling start temperature, the accelerated cooling rate, and the accelerated cooling stop temperature in the heat treatment immediately after the hot continuous rolling described above and further in the heat treatment performed after the hot continuous rolling are the top portion (reference numeral: 1) shown in FIG. ) And the head surface of the head corner (symbol: 2), or if the temperature is measured within a depth of 5 mm from the head surface, the entire rail head can be represented. By controlling the cooling rate, a fine pearlite structure with excellent wear resistance can be obtained.

In this manufacturing method, the refrigerant is not particularly limited, but air, mist, a mixed refrigerant of air and mist is used in order to ensure a predetermined cooling rate and to reliably control the cooling conditions at each part of the rail. Thus, it is desirable to perform predetermined cooling on the outer surface of each part of the rail.
The metal structure of the head of the steel rail manufactured by this manufacturing method is preferably a pearlite structure. However, depending on the selection of the component system and accelerated cooling conditions, a very small amount of proeutectoid ferrite structure in the pearlite structure. In some cases, a pro-eutectoid cementite structure and a bainite structure are formed. However, even if a small amount of these structures are formed in the pearlite structure, the fatigue strength and ductility of the rail are not greatly affected. It also includes a mixed ferrite structure, pro-eutectoid cementite structure and bainite structure.

Next, examples of the present invention will be described.
Table 1 shows the chemical composition of the test rail steel.
Table 2 shows the components of the rail manufactured by the rail manufacturing method of the present invention using the test rail steel shown in Table 1 (carbon content, PC value), hot rolling conditions, heat treatment conditions, and rail heads. The total elongation value of the microstructure, hardness, and tensile test is shown.
Table 3 shows the components (carbon content, PC value) of the rail manufactured by the comparative rail manufacturing method using the test rail steel shown in Table 1, hot rolling conditions, heat treatment conditions, and the microstructure of the rail head. , Hardness, total elongation of tensile test.

  Here, the drawings in this specification will be described. FIG. 1 shows the designation of each part of the rail. In FIG. 1, 1 is a top part and 2 is a head corner part. FIG. 2 illustrates test specimen collection positions in the tensile tests shown in Tables 2 and 3. FIG. 3 shows the relationship between the amount of carbon and the total elongation in the head tensile test results of the rail manufactured by the rail manufacturing method of the present invention shown in Table 2 and the rail manufactured by the comparative rail manufacturing method shown in Table 3. Is.

The configuration of the rail is as follows.
(1) Invention heat treatment rail (24)
Rails produced from rail steel within the above component range under hot rolling conditions and heat treatment conditions within the above limited range: reference numerals 1 to 4 and 6 to 15.
Rail which manufactured the rail steel within the said component range and PC value in the said range on the hot rolling conditions and heat processing conditions in the said limited range: Code | symbol 5, 16-24.
(2) Comparative heat treatment rail (16)
Rails manufactured with rail steel within the above component range under hot rolling conditions outside the above limited range: symbols 25 to 33.
Rail manufactured with rail steel within the above component range under heat treatment conditions outside the above limited range: 34-40.

Various test conditions are as follows.
(3) Head Tensile Test Tester: Universal Small Tensile Tester Test piece shape: Similar to JIS No. 4 Parallel part length: 25 mm, Parallel part diameter: 6 mm, Distance between elongation measurement grades: 21 mm
Test piece sampling position: 5mm below the rail head surface (see Fig. 2)
Tensile speed: 10 mm / min, test temperature: normal temperature (20 ° C.)

  As shown in Tables 2 and 3, the rail steel of the present invention (symbol: 1 to 24) has a maximum rolling pass time during continuous rolling compared to the comparative rail steel (symbol: 25 to 40). The austenite structure is refined by controlling the amount of carbon, the maximum rail head surface temperature during continuous rolling, to a value calculated by the formula consisting of the number of passes, and by performing heat treatment immediately after rolling. In addition, by performing heat treatment at an appropriate temperature range and cooling rate after rolling, fine pearlite that ensures wear resistance and ductility without generating proeutectoid cementite structure or martensite structure that adversely affects ductility. It can be an organization.

As shown in FIG. 3, the rail steel of the present invention (symbol: 1 to 24) is compared with the comparative rail steel (symbol: 25 to 33) that did not control the maximum time between rolling passes during continuous rolling. In terms of carbon content, the duct head has improved ductility. In addition to the control of the time between the maximum rolling passes during continuous rolling, V, Nb, and N are added according to the range of the PC value (formula 3) in order to suppress the growth of austenite grains generated after continuous rolling. The present invention rail steel (symbol: 5, 16 to 24) has a further improved ductility of the rail head at any carbon content.
Moreover, as shown in Table 2 and Table 3, this invention rail steel (code | symbol: 1-24) compared with the comparison rail steel (code | symbol: 34-40) which did not perform the heat processing after continuous rolling appropriately, Even in the amount of carbon, the duct head is secured.

The figure which showed the name in the head cross-section surface position of the rail manufactured with the rail manufacturing method of this invention. The figure which showed the test piece collection position in the tension test shown in Table 2 and Table 3. FIG. The present invention rail steel shown in Table 2 (V, Nb, N addition amount uncontrolled steel, code: 1-4, 6-15), the present invention rail steel (V, Nb, N addition amount controlled steel, code: 5, The figure which showed the relationship between the carbon amount and the total elongation value in the tension test result of the comparison rail steel (code | symbol: 25-40) shown in 16-24) and Table 3. FIG.

Explanation of symbols

1: Head part 2: Head corner part

Claims (5)

% By mass
C: more than 0.85 to 1.40% ,
Si: 0.05 to 2.00%,
Mn: 0.05 to 2.00%
In addition,
Cr: 0.05 to 2.00%,
Mo: 0.01 to 0.50%,
B: 0.0001 to 0.0050%,
Co: 0.003 to 2.00%,
Cu: 0.01 to 1.00%,
Ni: 0.01-1.00%,
Ti: 0.0050-0.0500%,
Mg: 0.0005 to 0.0200%,
Ca: 0.0005 to 0.0150%,
Al: 0.0100-1.00%,
Zr: 0.0001 to 0.2000%,
N: 0.0060-0.0200%,
V: 0.005-0.50%,
Nb: 0.002 to 0.050%
When manufacturing a rail from rail rolling steel slabs containing one or more of the following, and the balance Fe and unavoidable impurities, the number of consecutive passes is 2 to 30% with a cross-section reduction rate per pass of 2 to 30%. In finish rolling, the time between rolling passes (S, sec) is a value represented by the following formula 1 consisting of the carbon content (C, mass%) of steel and the maximum rail head surface temperature (T, ° C) during rolling. against (CPTl), method for producing high-carbon steel rail you and performing continuous rolling such that S ≦ CPTl.
CPT1 = 800 / (C × T) (1) % By mass
C: more than 0.85 to 1.40% ,
Si: 0.05 to 2.00%,
Mn: 0.05 to 2.00%
In addition,
Cr: 0.05 to 2.00%,
Mo: 0.01 to 0.50%,
B: 0.0001 to 0.0050%,
Co: 0.003 to 2.00%,
Cu: 0.01 to 1.00%,
Ni: 0.01-1.00%,
Ti: 0.0050-0.0500%,
Mg: 0.0005 to 0.0200%,
Ca: 0.0005 to 0.0150%,
Al: 0.0100-1.00%,
Zr: 0.0001 to 0.2000%,
N: 0.0060-0.0200%,
V: 0.005-0.50%,
Nb: 0.002 to 0.050%
When manufacturing a rail from a rail rolling steel slab comprising one or more of the following, and the balance Fe and inevitable impurities , the cross-section reduction rate per pass is 2 to 30%, and the number of passes is 3 times or more. In continuous finish rolling, the maximum time between rolling passes (S, sec) is the carbon content of steel (C, mass%), the maximum rail head surface temperature (T, ° C) during rolling, and the number of passes (P, times). for values of the formula 2 below consisting of (CPT2), method for producing high-carbon steel rail you and performing continuous rolling such that S ≦ CP T 2.
CPT2 = 2400 / (C × T × P) (Equation 2) The high-carbon steel rail according to claim 1 or 2, wherein the chemical composition of the steel rail has a value (PC) represented by the following formula 3 in a range of 0.30≥PC≥0.04. Manufacturing method.
PC = V (mass%) + 10 × Nb (mass%) + 5 × N (mass%) (Equation 3) The high carbon steel according to any one of claims 1 to 3 , wherein the rail head surface is accelerated and cooled to 950 to 750 ° C at a cooling rate of 5 to 30 ° C / sec immediately after the hot continuous rolling. Rail manufacturing method. After continuous hot rolling, from 700 ° C. or higher, the rail head surface accelerated cooling to at least 600 ° C. at a cooling rate 2 to 30 ° C. / sec, according to claim 1-4 then characterized by cool The manufacturing method of the high carbon steel rail of any one of Claims.